The characteristics of light transmitted through or reflected by a specimen are related to the color of the specimen. With the advent of instrumental analysis, the color of a specimen under test has become one of the most widely used bases for biochemical assay procedures. For example, transmittance type spectrophotometric measurements are normally made by simply directing light through a cuvette containing a fluid sample to be analyzed. A portion of the light beam is absorbed by the sample and the remaining portions pass through the cuvette to photodetector means for measurement. By comparing the measurement obtained from the sample with a measurement obtained from a control fluid the concentration of the analyzed sample can be calculated.
Similarly, reagent test devices used for qualitative or quantitative analysis of body fluids are normally contacted for a prescribed period of time with a body fluid, such as blood or urine. The reflectance of the reacted test device will vary depending on the concentration of the analyte in the body fluid being examined. Thus, by photoelectrically measuring light reflected from the test device the desired analysis can be made by correlation of reflectance to reflectances obtained from known concentrations.
It is known that often times more information can be obtained about the analyte being measured if measurements are made at more than one wavelength. In addition, the accuracy of such measurements can be improved by taking measurements at more than one wavelength since such a procedure permits one to eliminate or reduce the influence of analyte interferences present at one wavelength and not another wavelength.
Spectrophotometers or reflectance photometers which have the capability of measuring light sequentially at different wavelengths are inherently subject to all of the errors which occur in making sequential measurements. Accordingly, a need has existed in the art for a method and device capable of making rapid, accurate, reproducible light measurements simultaneously at more than one wavelength.
The device and method of the present invention not only permit transmittance or reflectance measurements to be made simultaneously at more than one wavelength, but the method and apparatus permit the wavelengths at which measurements are made to be selected accurately, quickly and simply by rotating arms relative to the center of a circularly variable filter. Alternatively, the circularly variable filter can be rotated about its center.